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It’s a weakness of SIP that it doesn’t support the alternative-WCS convention, and this has led to real problems in deciding what Astropy should do with it when there are alternative WCSes defined.</div>
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For cases where the convention actually supports the alternative-WCS “a” convention (as in the question in this thread) I think it’s very clear that the consistent intent of the design of FITS was to have each of the 27 possible WCSes be independent of the
others.</div>
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There are cases, such as for SPHEREx, where you absolutely don’t want optical distortions to “leak” into other WCSes. In SPHEREx, we use a FITS-standard (though rather exotic) 2D-lookup-table WCS to document the (intentionally) varying spectral bandpass across
the field of view. That bandpass is tied to the physical pixels, not to the sky, created by linear-variable filters immediately in front of the detectors, so including optical distortions from the primary spatial WCS in the spectral WCS would be a serious
error.</div>
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We have to leave it up to projects/missions to choose which WCSes to apply distortions to; if that means they have to repeat distortion models for multiple “a” values, well, within the limitations of FITS: so be it.</div>
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Best wishes,</div>
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Gregory Dubois-Felsmann</div>
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Caltech/IPAC</div>
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NSF/DOE Vera C. Rubin Observatory Science Platform science lead, Data Engineering lead</div>
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NASA SPHEREx mission pipeline system designer</div>
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<span style="font-family: -apple-system, HelveticaNeue; font-size: 13.398001px; color: rgb(33, 33, 33);">--</span><span style="font-family: Aptos, Aptos_MSFontService, -apple-system, Roboto, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);"><br>
</span><span style="font-family: -apple-system, HelveticaNeue; font-size: 13.398001px; color: rgb(33, 33, 33);">Gregory Dubois-Felsmann, Ph.D. | Senior Staff Scientist | Caltech/IPAC</span><span style="font-family: Aptos, Aptos_MSFontService, -apple-system, Roboto, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);"><br>
</span><span style="font-family: -apple-system, HelveticaNeue; font-size: 13.398001px; color: rgb(33, 33, 33);">Science Platform Scientist, Vera C. Rubin Observatory</span><span style="font-family: Aptos, Aptos_MSFontService, -apple-system, Roboto, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);"><br>
</span><span style="font-family: -apple-system, HelveticaNeue; font-size: 13.398001px; color: rgb(33, 33, 33);">Pipeline System Designer, NASA SPHEREx mission</span><span style="font-family: Aptos, Aptos_MSFontService, -apple-system, Roboto, Arial, Helvetica, sans-serif; font-size: 12pt; color: rgb(0, 0, 0);"><br>
</span><span style="font-family: -apple-system, HelveticaNeue; font-size: 13.398001px; color: rgb(33, 33, 33);">Mail Code MR 100-22 | Pasadena, CA 91125-2200 | </span><span style="font-family: -apple-system, HelveticaNeue; font-size: 13.398001px; color: rgb(0, 120, 212);">gpdf@ipac.caltech.edu</span></div>
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<div id="divRplyFwdMsg" dir="ltr"><font face="Calibri, sans-serif" style="font-size:11pt" color="#000000"><b>From:</b> fitsbits <fitsbits-bounces@listmgr.nrao.edu> on behalf of Thompson, William T. (GSFC-671.0)[ADNET SYSTEMS INC] via fitsbits <fitsbits@listmgr.nrao.edu><br>
<b>Sent:</b> Tuesday, 07 July 2026 12:43:25<br>
<b>To:</b> fitsbits@listmgr.nrao.edu <fitsbits@listmgr.nrao.edu><br>
<b>Subject:</b> [fitsbits] Question about distortions when multiple coordinate systems are applied {External}</font>
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Folks:</div>
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I’ve been presented with a question about the use of distortion parameters when a FITS header contains multiple coordinate systems. Earlier I had asked about this issue when using the Simple Image Polynomial convention, but now I’m asking about distortions
as described in the Calabretta et al. 2004 paper.</div>
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Specifically I’m asking about <i>prior</i> (DP) distortions, since subsequent (DQ) distortions are a different case entirely. I’ve been told that astropy will automatically apply distortions encoded in DPj keywords when there are no DPja keywords for alternative
coordinate system “a”. I’ve looked through the Calabretta et al. 2004 paper, and cannot find anything that says that this substitution should be made. Is this a convention that is widely adopted?</div>
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To me, it does make sense that any prior distortion parameters applicable to one coordinate system would be equally applicable to any alternate coordinate system. The same is not necessarily true for subsequent distortion parameters, given the possibly different
effects from the CRPIXja and CDi_ja values between the systems.</div>
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I’d appreciate any clarity that can be applied to this situation. If that convention is widely used, then I should adopt it too.</div>
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Thank you,</div>
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Bill Thompson</div>
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<b>William Thompson</b></div>
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